1. Field of the Invention
This invention relates to a method for simultaneously sweetening and drying a gas stream.
More particularly, this invention relates to a process in which a high boiling, organic liquid is contacted under pressure with a natural gas stream to absorb the hydrogen sulfide and water without substantially altering the carbon dioxide or hydrocarbon content and is continuously regenerated.
2. Description of the Prior Art
The present practice is to condition produced, sour, natural gas for sale to a pipeline in two, separate processes. The first, and more severe, of the requirements is to reduce the sour, hydrogen sulfide, content to 1/4 grain per 100 scf, or 4 ppm.
In a common method for sweetening gas, an aqueous solution of an amine and the sour gas are contacted counter-currently on a plurality of trays in a tower. Hydrogen sulfide forms a loose, addition compound with amines.
Thermal decomposition of the compound so formed occurs in a second trayed column equipped with a reboiler and condenser. The bottoms from the absorption column are fed to the regeneration column. The purified amine is withdrawn from the reboiler for continuous contact with the gas.
One of the problems with this process is that most amines also react with the carbon dioxide in the natural gas. Because the addition compound with carbon dioxide must also be decomposed, it is necessary to increase the circulation rate of the amine, the size of the regeneration column, and the reboiler heat load.
The second of the requirements for the produced natural gas is to reduce the water content to 5 pounds per MMscf, or 110 ppm. Dehydration is effected in a process very similar to that for sweetening. The main difference is the use of a glycol rather than an amine.
Glycol dehydration is practiced widely because of the moderate capital investment and the low operating cost. However, sweetening with an amine plant requires a considerable capital expenditure. There are many sour, natural gas wells for which the value of the produced gas is insufficient to justify the investment required for an amine system. For years, the iron sponge process has been the only economic way to sweeten such wells. While iron sponge does selectively remove hydrogen sulfide without absorbing carbon dioxide, the operation is so troublesome and the labor cost is so high, that many of these wells have not been produced.
Recently, a process has been developed using a slurry of zinc oxide in a zinc acetate solution to selectively absorb the hydrogen sulfide in the natural gas. The capital cost is relatively low, but the operating cost, though moderate, still limits the applicability to wells where the amount of hydrogen sulfide to be removed is not great.
While the zinc process certainly appears to be a very attractive alternative to the iron sponge process, both are best suited and, to an extent, limited to low pressure applications where a large contacting vessel can be constructed inexpensively.
A combination of monethanolamine and diethylene glycol has been used to dehydrate and sweeten natural gas. U.S. Pat. Nos. 2,177,068; 2,515,752 and 2,547,278 disclose such a process. In the disclosed system, the absorption tower is divided into two sections, so that the gas can be contacted successively with different aqueous solutions of the above liquids. These absorbents, or solvents, which first sweeten and then dry the gas, are regenerated separately in different reboilers. The flow pattern is fairly complicated, and obtaining good heat exchange between the streams makes the operation difficult to control. Also, after absorbing the hydrogen sulfide, the aqueous solution is quite corrosive. Furthermore, the capital investment required for this process is quite substantial.
N-methyl-2-pyrrolidone (M Pyrol) has been used as a sweetening solvent in the Purisol process. The Purisol process as disclosed in U.S. Pat. Nos. 3,324,627 and 3,120,933 uses three towers: absorber, reabsorber, and regenerator. The high pressure gas stream enters the bottom of the absorber and flows upward counter-currently to the lean M Pyrol which is introduced at the top of the absorber. After absorbing the hydrogen sulfide and water, the M Pyrol leaves the absorber and enters the intermediate pressure reabsorber. Some of the hydrogen sulfide and the dissolved hydrocarbons are flashed from the rich solvent in the bottom of the reabsorber and flow upward in counter-current contact with a portion of the regenerated solvent which enters the top tray of the reabsorber. The effluent gas from the reabsorber is used for plant fuel. The rich solvent together with the solvent fed to the top of the reabsorber, flows through a heat exchanger to the low pressure regeneration column. The hydrogen sulfide leaves at the top of the regeneration column, and the purified M Pyrol is pumped from the bottom of the regeneration column through the heat exchanger to the absorber and reabsorber columns.
There is a real need for a new sweetening process that will be suitable for high pressure, relatively low flow rate, gas streams provided that the capital outlay is reasonable and the operating cost is moderate. If the process can also dry the gas, the economic justification increases substantially -- by both the investment and opeational cost of a glycol dehydrator.
An absorbent with a high selective affinity for hydrogen sulfide is required. Substantial rejection of carbon dioxide is necessary to prevent a prohibitively high flow rate when the sour gas stream to be treated also contains an appreciable amount of carbon dioxide.
A high affinity for water by the same absorbent is also required. But the presence of water must not significantly reduce the affinity of the absorbent for hydrogen sulfide.
Substantial rejection of hydrocarbons by the absorbent is required to limit the losses of the natural gas stream and the contamination of higher boiling hydrocarbons.
The absorbent must be thermally stable up to its boiling point. A high boiling point is required to reduce the absorbent losses and to facilitate the regeneration of the absorbent with heat.
Safety demands that the absorbent be non-toxic. It is highly desirable that the material not irritate the skin when it is handled.
Good heat transfer and fluid flow properties are very desirable for the absorbent. It should have a high thermal conductivity, a low specific heat and a low viscosity.
Finally, the absorbent must be non-corrosive to construction materials such as mild steel and aluminum.